In a spark-ignition engine the load is generally controlled by a throttle valve provided in an intake tract. By adjusting the throttle valve it is possible to reduce the pressure of the intake air downstream of the throttle valve to a greater or lesser degree. In this way it is possible, for a constant combustion chamber volume, to adjust the air mass, that is to say the quantity of air, via the pressure of the intake air. Low loads use heavier throttling, that is to say extensive pressure reduction in the intake tract, so that large charge cycle losses have to be accepted, particularly in the partial-load range. In this respect the fuel-air mixture in the cylinder has a lower density at low loads than at higher loads.
The load-dependent variation of the aircharge density can present problems when initiating the spark-ignition. In order to ensure reliable ignition of the fuel-air mixture at low loads, that is to say, with a less dense mixture, the electrodes of the spark plug may have a relatively large gap, in order to form a large spark path for the ignition spark. A large electrode gap uses a high ignition voltage.
If the fuel-air mixture then becomes denser as the load increases, the voltage used in order to initiate reliable ignition is also increased, since the denser fuel-air mixture present between the electrodes has an insulating effect on the crossing of the ignition spark from one electrode to the other.
The high ignition voltage used can damage the spark plug itself and/or other components, in particular current supply components carrying a high voltage, that is to say components of the electrical system of the motor vehicle. In the event of an excessive ignition voltage demand, arcing may occur between the high-voltage leads and the chassis ground. No ignition spark is formed between the electrodes and misfiring may occur.
Besides the density of the mixture, the air/fuel ratio λ also has an influence on the initiation of the spark-ignition, in particular, however, on the combustion induced by ignition sparks. The fuel-air mixture of a spark-ignition engine can be enriched or made leaner as a function of the instantaneous load T prevailing, an enrichment preferably being undertaken, that is to say the air/fuel ratio λ being reduced, as the load increases.
Enrichment (λ<1) frequently ensues whenever high exhaust gas temperatures are to be anticipated, that is to say at high loads, in order to prevent a thermal overload of individual components of the internal combustion engine. This is done by injecting more fuel than can be burned with the quantity of air delivered, the excess fuel likewise being heated and vaporized, so that the temperature of the combustion gases falls. This approach is to be regarded as disadvantageous from various energy standpoints, particularly with regard to the fuel consumption of the internal combustion engine, and with regard to the pollutant emissions, but is recognized as admissible and practical.
Making the fuel-air mixture leaner as the load diminishes is a measure aimed at dethrottling of the internal combustion engine, less fuel being injected than could be burned with the quantity of air delivered, that is to say more fresh air is introduced into the cylinders than is necessary for combustion of the fuel introduced, thereby diluting the mixture. These variations in aircharge density and content may make spark initiation difficult.
The inventors have recognized the above mentioned problems and herein disclose a method for a spark ignition engine capable of initiating combustion via auto-ignition when it is advantageous. In the method according to the disclosure account is taken of the various load-specific demands of the spark plug in that the ignition of the fuel-air mixture is initiated either by spark-ignition or by auto-ignition—preferably as a function of load, T. In order to initiate auto-ignition, exhaust gas discharged from one cylinder into the exhaust discharge system, is introduced into another cylinder, in order to induce auto-ignition by this ignition gas.
A method is disclosed for operating a spark ignition engine, comprising: discharging exhaust gases from each of at least two cylinders via an exhaust system comprising exhaust pipes from the at least two cylinders coming together to form an exhaust manifold; and initiating compression ignition with exhaust gas from a first, exhaust-gas-supplying cylinder, which is discharged from the first cylinder into the exhaust system, the exhaust gas is introduced into an adjacent, exhaust-gas-receiving cylinder via the exhaust system. The hot exhaust gas from the first cylinder does not mix completely with the fresh charge of the adjacent cylinder. During the subsequent compression stroke the trapped hot exhaust gas is being compressed and its temperature further increased such that it can serve as ignition initiator in the same way like a spark plug does. Thus in contrast and in delimitation to the known auto-ignition this method provokes a more spark-ignition-like slower combustion and is therefore suitable for high loads. Additionally, the method of the present disclosure may be adapted such that spark ignition is still active under conditions for autoignition.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Further, the inventors herein have recognized the disadvantages noted herein, and do not admit them as known.